Liquid crystal displays (LCDs) are widely used in portable televisions, laptop personal computers, notebooks, electronic watches, calculators, mobile phones and office automation devices due to their advantages of small size, light weight, low driving voltage, low power consumption and good portability.
LCDs are generally classified as two major types: transmissive LCDs and reflective LCDs. A transmissive LCD requires backlight, while a reflective LCD reflects environmental light. So far, reflective LCDs are relatively popular. Since the reflective LCDs require high levels of ambient light to be readable, they work very well in bright sunlight but do not perform well indoors. In order to perform well in low light environments, a so-called “transflective” LCD has been developed in recent years.
The transflective LCD is designed as a combination of the transmissive and the reflective designs. Please refer to the cross-sectional structure of FIG. 1(a). A pixel cell of a conventional transflective LCD comprises a common electrode 10, a transmissive electrode 11, a reflective electrode 12 and a liquid crystal layer 13. The liquid crystal layer 13 contains a plurality of liquid crystal molecules and is sandwiched between the common electrode 10, the transmissive electrode 11 and the reflective electrode 12. With such structure, each pixel cell of the transflective LCD can be referred to comprise two portions. One is a transmissive portion consisting of the common electrode 10, the liquid crystal layer 13 and the transmissive electrode 11, whereas the other one is a reflective portion consisting of the common electrode 10, the liquid crystal layer 13 and the reflective electrode 12. The liquid crystal molecules are aligned according to driving voltages applied between the electrodes. The light passing through the liquid crystal layer 13 is controlled accordingly. FIG. 1(b) is a schematic circuit diagram illustrating a cell of the transflective LCD shown in FIG. 1(a).
FIGS. 2(a) and 2(b) are brightness vs. voltage plots of the reflective portion and the transmissive portion, respectively. As shown in FIG. 2(a), for the reflective portion, a maximum brightness is achieved at just about 3.5 volts of the applied voltage. Whereas, as shown in FIG. 2(b), the maximum brightness for the transmissive portion can be obtained when the applied voltage is approximately 5 volts. The symbols L1 in FIGS. 2(a) and 2(b) indicate a specific driving voltage giving a brightness index of 100%, while the symbols L2, L3, . . . indicate other driving voltages resulting in various brightness. It is understood from the figures while the driving voltages applied for the transmissive portion and the reflective portion are the same, the required operation voltages by the transmissive portion and the reflective portion, however, are different due to the difference in operation principles. Since the brightness varies with the driving voltages, different criteria for determining brightness levels and thus uneven brightness effects for the reflective portion and the transmissive portion are rendered.